As a method of making color correction for improving a color reproduction effect in a color reproduction process of a printer or printing press, a color masking method of obtaining data on an output color space by making matrix operations of data on an input color space, a method of converting data on an input color space into data on an output color space using a lookup table (LUT), and the like are prevalently used.
However, the output characteristics of a color printer or printing press exhibit strong nonlinearity. Therefore, in a global method such as the color masking method, i.e., a color correction method in which a change in element of a matrix influences the overall output color space, the characteristics of a color printer or printing press cannot be satisfactorily approximated in the entire color gamut. Also, in the method using a LUT, table values are normally obtained by the color masking method, and difficulty in color reproduction remains unsolved.
In general, most of color printers form images using four, i.e., C, M, Y, and K color agents. Images formed by these four color agents look different depending on the way black (K) is generated, although they exhibit the same colorimetric values. Various black generation methods are known. However, most of these methods must change conditions depending on the types of images, the tone characteristics of a printer, and the like, and it is difficult to easily adjust the way black is generated.